Method of fabricating light emitting device and thus-fabricated light emitting device

ABSTRACT

A light emitting device wafer having a light emitting layer section  24  having an AlGaInP-base double heterostructure, and a GaP light extraction layer  20  disposed on the light emitting layer section so as to allow a first main surface thereof to compose a first main surface of the wafer is fabricated so that the first main surface of the GaP light extraction layer appears as the ( 100 ) surface. The first main surface of the GaP light extraction layer  20  composed of the ( 100 ) surface is etched using an etching solution for surface roughening to thereby form surface roughening projections  40   f . Accordingly, there can be provided a method of fabricating a light emitting device having the GaP light extraction layer agreed with the ( 100 ) main surface, capable of readily subjecting the ( 100 ) main surface to surface roughening.

RELATED APPLICATIONS

This application claims the priority of Japanese Patent Application No.2004-131806 filed on Apr. 27, 2004, which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method of fabricating a light emittingdevice and thus-fabricated light emitting device.

2. Description of the Related Art

Light emitting device having a light emitting layer section thereofcomposed of an (Al_(x)Ga_(1-x))_(y)In_(1-y)P alloy (where, 0≦x≦1, 0≦y≦1;simply referred to as AlGaInP alloy, or more simply as AlGaInP,hereinafter) can be realized as a high-luminance device over a widewavelength range typically from green region to red region, by adoptinga double heterostructure in which a thin AlGaInP active layer issandwiched between an n-type AlGaInP cladding layer and a p-type AlGaInPcladding layer, both having a larger band gap. Current is supplied tothe light emitting layer section through a metal electrode formed on thesurface of the device. The metal electrode acts as a light interceptor,so that it is formed, for example, so as to cover only the centerportion of a first main surface of the light emitting layer section, tothereby extract light from the peripheral region having no electrodeformed thereon.

In this case, a smaller area of the metal electrode is advantageous interms of improving the light extraction efficiency, because it canensure a larger area for the light leakage region formed around theelectrode. Conventional efforts have been made on increasing the energyof light extraction by effectively spreading current within the devicethrough consideration on geometry of the electrode, but increase in theelectrode area is inevitable anyhow in this case, having been fallen ina dilemma that a smaller light extraction area results in a limitedenergy of light extraction. Another problem resides in that the currentis less likely to spread in the in-plane direction, because the dopantcarrier concentration, and consequently the conductivity ratio, of thecladding layer is suppressed to a slightly lower level in order tooptimize emissive recombination of carriers in the active layer. Thisresults in concentration of the current into the region covered by theelectrode, and consequently lowers the substantial energy of lightextraction from the light leakage region. There has been adopted amethod of forming, between the cladding layer and the electrode, alow-resistivity GaP light extraction layer having a dopant concentrationlarger than that of the cladding layer. The GaP light extraction layerincreased in the thickness to a certain degree is not only successful inimproving the in-plane current spreading effect, but also in increasingextractable energy of light from the side faces of the layer to therebyraise the light extraction efficiency. It is necessary for the lightextraction layer to be formed using a material having a band gap energylarger than a light quantum energy of the beam of emitted light, for thepurpose of efficient transmission of the beam of emitted light andraising the light extraction efficiency. In particular, GaP is widelyused for composing the light extraction layer of AlGaInP-base lightemitting device, by virtue of its large band gap energy and smallabsorption of the beam of emitted light.

In thus-configured light emitting device using the peripheral region ofthe metal electrode on the first main surface of the light extractionlayer as the light extraction area, not all components of the emittedlight directed from inside of the device towards the light extractionarea can be extracted, because some components incident on the lightextraction area at an angle (angle of incidence herein means an angle ofthe direction of incidence of beam away from the normal line on thearea) larger than the critical angle returns back inside the device bytotal reflection. Japanese Laid-Open Patent Publication “Tokkai” Nos.2003-218383 and 2003-209283 disclose techniques of roughening (alsoreferred to as frosting) the first main surface of the light extractionlayer using an appropriate etching solution so as to form fineirregularities, aiming at reducing probability of incidence of the beamof emitted light at large angles and at consequently raising the lightextraction efficiency.

Japanese Laid-Open Patent Publication “Tokkai” No. 2003-218383, however,discloses that the surface roughening using the etching solution canroughen some surfaces but cannot roughen other surfaces depending onorientation of the exposed surface, and is therefore not alwayseffective in terms of upper surface roughening, so that improvement inthe light extraction efficiency is achievable only to a limited degree,and further improvement in the luminance is not easy. Japanese Laid-OpenPatent Publication “Tokkai” No. 2003-209283 more specifically disclosesthat “the main surface of semiconductor substrate generally shows the(100) surface or a surface several degrees off-angled from the (100)surface, so that the surface of any of the individual semiconductorlayers grown thereon has also the (100) surface or a surface severaldegrees off-angled from the (100) surface, wherein it is difficult toroughen the (100) surface and the surface several degrees off-angledfrom the (100) surface”. The light extraction layer disclosed inJapanese Laid-Open Patent Publication “Tokkai” No. 2003-209283 is aGaAlAs layer, whereas Japanese Laid-Open Patent Publication “Tokkai” No.2003-218383 discloses a GaP light extraction layer, having again the(100) surface exposed on the first main surface thereof.

Putting all aspects disclosed in Japanese Laid-Open Patent Publication“Tokkai” Nos. 2003-218383 and 2003-209283 together, it is obvious thatthe GaP light extraction layer having the (100) surface on the firstmain surface thereof cannot be roughened simply by immersing it into theetching solution, so far as any publicly-known etching solution for GaP(hydrochloric acid, sulfuric acid, hydrogen peroxide or mixed solutionsof these components, according to paragraph 0026 in Japanese Laid-OpenPatent Publication “Tokkai” No. 2003-218383) is used as the etchingsolution, and that it is difficult to form the irregularities capable ofimproving the light extraction efficiency to a sufficient degree.

Japanese Laid-Open Patent Publication “Tokkai” No. 2003-218383discloses, as one specific solution of this problem, a method of etchingthe (100) main surface of the GaP light extraction layer, after coveringit with a finely-patterned resin mask. Although Japanese Laid-OpenPatent Publication “Tokkai” No. 2003-218383 also formally suggests wetetching (chemical etching) as the etching method, all specificdisclosures including the embodiments are made only on dry etching basedon RIE (reactive ion etching) which costs high, and is disadvantageousin terms of its extremely low process efficiency, because only a smallarea of substrate can be processed at a time. As far as the presentinventors investigated, it is also supposed that the chemical etchingusing hydrochloric acid, sulfuric acid, hydrogen peroxide, or any mixedsolution of these components causes a considerable intrusion of theetching under the mask, and therefore cannot form the distinctirregularities on the GaP light extraction layer as shown in JapaneseLaid-Open Patent Publication “Tokkai” No. 2003-209283.

On the other hand, Japanese Laid-Open Patent Publication “Tokkai” No.2003-209283 gives no specific information on the formation of theirregularities by etching the (100) main surface of the GaP lightextraction layer, because the light extraction surface herein iscomposed of GaAlAs. A method adopted herein is such as forming amacroscopic secondary trench pattern having a triangle section bymechanical processing so as to expose the (111) surface allowing theetching to proceed thereon more smoothly, and subjecting the surface ofthe secondary pattern to chemical etching. The method is, however,disadvantageous in that the number of process steps increasescorresponding to necessity of the mechanical processing for forming thetrenches.

The light emitting device having the GaP light extraction layer canincrease the energy of light extractable from the side faces of the GaPlight extraction layer, if the layer is thickened. Therefore the lightextraction efficiency of the device as a whole can further be increasedby roughening also the side faces of thus-thickened GaP light extractionlayer. The methods of surface roughening described in Japanese Laid-OpenPatent Publication “Tokkai” Nos. 2003-218383 and 2003-209283, however,essentially need process steps of forming the mask and the trenches,which are permissible only on the main surface of the wafer, and thisconsequently raises a critical disadvantage in that the surfaceroughening is not adoptable to the side faces of the chip which appearonly after the wafer is diced. In particular in the dry etching such asRIE as described in Japanese Laid-Open Patent Publication “Tokkai” No.2003-218383, it may be absolutely impossible to etch the side faces byallowing the etching beam, directed straightforward to the main surfaceof the layer, to come round onto the side faces, due to strongdirectionality of the etching beam.

It is therefore a subject of this invention to provide a method offabricating a light emitting device, having a GaP light extraction layerhaving the (100) surface as the main surface thereof, capable of readilyroughening the (100) main surface, and a light emitting device having anexcellent light extraction efficiency realizable only by this method.

SUMMARY OF THE INVENTION

Aiming at solving the above-described subject, a method of fabricating alight emitting device according to this invention is such as having:

a light emitting device wafer fabricating step fabricating a lightemitting device wafer having a light emitting layer section based on adouble heterostructure in which a first-conductivity-type claddinglayer, an active layer and an second-conductivity-type cladding layer,each of which being composed of a compound semiconductor having acomposition allowing lattice matching with GaAs, out of compoundsemiconductors expressed by formula (Al_(x)Ga_(1-x))_(y)In_(1-y)P(where, 0≦x≦1, 0≦y≦1), are stacked in this order, and a GaP lightextraction layer disposed on the light emitting layer section so as toallow a first main surface thereof to compose a first main surface ofthe wafer, so that the first main surface of the GaP light extractionlayer appears as the (100) surface;

a main light extraction area roughening step forming surface rougheningprojections on the first main surface of the GaP light extraction layercomposed of the (100) surface, by etching the surface using an etchingsolution for surface roughening containing acetic acid, hydrofluoricacid, nitric acid, iodine and water up to a total content of 90% by massor more, and having a total content by mass of acetic acid, hydrofluoricacid, nitric acid and iodine larger than the content by mass of water;and

a dicing step dicing the light emitting device wafer to therebyfabricate light emitting device chips each having the surface rougheningprojections on the first main surface of the GaP light extraction layer.

According to the method of this invention using the invention-specificetching solution for surface roughening containing acetic acid,hydrofluoric acid, nitric acid and iodine, formation of irregularitiesbased on the principle of anisotropic etching can proceed in adistinctive manner on the first main surface of the GaP light extractionlayer composed of the (100) surface, simply by allowing the first mainsurface to contact with the etching solution, without special need ofmasking or the like, and thereby the surface roughening projections canbe formed on the first main surface of the GaP light extraction layer inan efficient and inexpensive manner. The total content of acetic acid,hydrofluoric acid, nitric acid, iodine and water is 90% by mass or more,wherein any content lower than this value fails in efficiently formingthe surface roughening projections. The total content of acetic acid,hydrofluoric acid, nitric acid and iodine smaller than the content ofwater also similarly results in inefficient formation of the surfaceroughening projections. The residual portion remaining after subtractingthe total content of acetic acid, hydrofluoric acid, nitric acid, iodineand water from 100% by mass may be occupied by other components(carboxylic acid other than acetic acid, for example), so far as theanisotropic etching effect with respect to GaP on the (100) surface isnot impaired.

“A compound semiconductor allowing lattice matching with GaAs” in thisinvention means a compound semiconductor having a ratio of latticemismatching expressed by {|a1−a0|/a0}×100(%) fallen within an 1% range,where a1 is a lattice constant of the compound semiconductor expectedfor the bulk crystal state having no stress-induced lattice displacementproduced therein, and a0 is a lattice constant of GaAs in the samestate. “A compound semiconductor having a composition allowing latticematching with GaAs, out of compound semiconductors expressed by formula(Al_(x′)Ga_(1-x′))_(y′)In_(1-y′)P (where, 0≦x′≦1, 0≦y′≦1)” will bereferred typically to as “AlGaInP lattice-matched to GaAs”. The activelayer may be configured as a single AlGaInP layer, or as a quantum welllayer having barrier layers and well layers, differed in the AlGaInPcomposition from each other, alternately stacked therein (quantum welllayers as a whole is assumed as a single active layer).

The fact that the (100) surface appears on the first main surface of theGaP light extraction layer means, in a narrow sense, that the principalcrystal axis of the GaP light extraction layer is agreed with the [100]direction of GaP crystal, whereas it is to be understood in thisinvention that also inclination of the principal crystal axis up to 25°or smaller (more preferably 15° or smaller; and 1° or larger in view ofintensifying the effect of off-angle) away from the [100] direction(that is, provision of off-angle) will be included in the concept of“the (100) surface appears on the first main surface of the GaP lightextraction layer”. Although the first main surface in this case willhave a higher order of surface index in a strict sense, the paragraphsbelow may denote the first main surface off-angled from the (100)surface as (100)_(OFF), and the first main surface agreed with the (100)surface as (100)_(J), as the occasion demands so as to avoid complexityin the explanation. It is also to be understood that any preferablesurfaces adopted herein expressed by the Miller index {hkl} representsurfaces inclined 1° to 25°, both ends inclusive, away from the exact{hkl}_(J) surface will be represented by the index, unless otherwisespecifically be noted, and the notation {hkl}_(OFF) will be used ifdiscrimination is necessary.

In order to enable formation of the surface roughening projections bychemical etching on the surface of a compound semiconductor singlecrystal having no crystal boundary, it is necessary for the etchingsolution employed herein to exhibit larger etchrate on a crystal surfacehaving a specific orientation than on crystal surfaces having otherorientations (the surface advantageous for the etching will be referredto as “predominantly-etchable surface”, hereinafter), that is, to enableanisotropic etching dependent on surface orientation. Crystal surfaceobtained after allowing anisotropic etching to proceed thereon shows acombination of several predominantly-etchable surfaces differed in thesurface index but equivalent in terms of crystallography, and thereforeproduces irregularity ascribable to the geometry specific to the crystalstructure. GaP based on the cubic system has the surfaces which belongto the {111} group showing the closest packing as thepredominantly-etchable surface. Assuming that also the surface havingthe inverse sign as the identical surfaces, the {111} group is nowunderstood as including four surfaces differed in the orientation, sothat the surface roughening based on anisotropic etching tends toproduce pyramid-like irregularities based on combination of thesesurfaces.

The first main surface of the GaP light extraction layer adopted in thisinvention appears as the (100) surface largely inclined (approximately55° for the exact (100) surface) from the predominantly-etchable (111)surface, and can allow the formation of irregularities to distinctivelyproceed thereon, if the predominantly-etchable surface can selectivelybe exposed in the initial stage of the etching. The etching solution forsurface roughening adopted in this invention not only shows a largeetchrate to a certain degree on the (100) surface of GaP, but also showsan etchrate on the (111) surface appropriately differed therefrom, andconsequently has a large effect of forming the irregularities whileallowing the (111) surface to selectively expose. The chemical etchingsolution disclosed in Japanese Laid-Open Patent Publication “Tokkai” No.2003-218383 (hydrochloric acid, sulfuric acid, hydrogen peroxide ormixed solutions of these components) is unsuccessful in appropriatelyforming the surface roughening projections on the (100) surface,supposedly because the solution shows only an extremely small etchrateon the (100) surface so that the etching hardly proceeds from its earlystage, or conversely the etchrate on the (100) surface is too close tothe etchrate on the (111) surface, so that the {111} group cannotdistinctively be exposed even under progress of the etching.

It is preferable to adopt the etching solution such as containing:

acetic acid (on CH₃COOH basis): 37.4% by mass or more and 94.8% by massor less;

hydrofluoric acid (on HF basis): 0.4% by mass or more and 14.8% by massor less;

nitric acid (on HNO₃ basis): 1.3% by mass or more and 14.7% by mass orless; and

iodine (on I₂ basis): 0.12% by mass or more and 0.84% by mass or less,

and has a water content of 2.4% by mass or more and 45% by mass or less.Any of these components out of the above-defined ranges results ininsufficient effect of anisotropic etching on the (100) surface of GaPsingle crystal, and thereby results in only insufficient formation ofthe surface roughening projections on the first main surface of the GaPlight extraction layer. The surface roughening etching solution morepreferably adoptable is such as containing:

acetic acid (on CH₃COOH basis): 45.8% by mass or more and 94.8% by massor less;

hydrofluoric acid (on HF basis): 0.5% by mass or more and 14.8% by massor less;

nitric acid (on HNO₃ basis): 1.6% by mass or more and 14.7% by mass orless; and

iodine (on I₂ basis): 0.15% by mass or more and 0.84% by mass or less,

and has a water content of 2.4% by mass or more and 32.7% by mass orless. In other words, in view of raising the anisotropic etching effecton the (100) surface of GaP single crystal, it can be said as importantto suppress the water content to a low level as described in the above,and to allow acetic acid, rather than water, to function as a mainacidic solvent.

For the case where the GaP light extraction layer is formed to as thickas 10 μm or more in the method of this invention, it is allowable toadditionally carry out a side-face light extraction area roughening stepforming surface roughening projections on the side-face light extractionareas of the GaP light extraction layer composed of the side faces ofthe chips formed by the dicing, by etching the areas using the etchingsolution for surface roughening.

A light emitting device of this invention can be realized only by theabove-described method, characterized in having:

a light emitting layer section based on a double heterostructure inwhich a first-conductivity-type cladding layer, an active layer and ansecond-conductivity-type cladding layer, each of which being composed ofa compound semiconductor having a composition allowing lattice matchingwith GaAs, out of compound semiconductors expressed by formula(Al_(x)Ga_(1-x))_(y)In_(1-y)P (where, 0≦x≦1, 0≦y≦1), stacked in thisorder; and

a GaP light extraction layer having a thickness of 10 μm or more, formedon the first main surface side of the light emitting layer section,having a part of a first main surface thereof covered with alight-extraction-side electrode, allowing an area of the first mainsurface not covered by the light-extraction-side electrode to functionas a main light extraction area, and allowing the side face areas tosimilarly function as side-face light extraction areas,

wherein the GaP light extraction layer is a GaP single crystal layerhaving the (100) surface as the first main surface, and has surfaceroughening projections formed by etching on both of the main lightextraction area and the side-face light extraction areas.

The fact that the surface roughening projections can be formed on theGaP light extraction layer by virtue of the anisotropic etching effectsimply by immersing the layer into an etching solution means that thesurface roughening projections can readily be formed also on the sidefaces of the GaP light extraction layer on which the mask formation andthe trench formation, described in Japanese Laid-Open Patent PublicationNos. 2003-218383 and 2003-209283, are intrinsically not available. Inparticular for the case where the GaP light extraction layer is formedto as thick as 10 μm or more, formation of the surface rougheningprojections on the side faces thereof can considerably raise the lightextraction efficiency of the device, synergistically with an effect ofincrease in the side face area by virtue of increase in the thickness ofthe GaP light extraction layer. This effect is said to be neverachievable by the conventional techniques disclosed in JapaneseLaid-Open Patent Publication Nos. 2003-218383 and 2003-209283. In viewof raising the light extraction efficiency from the side faces of theGaP light extraction layer, it is more preferable to adjust thethickness of the GaP light extraction layer to 40 μm or more (the upperlimit is set to 200 μm, for example).

The outer surfaces of the surface roughening projections can be formedmainly as having the {111} surfaces by chemical anisotropic etching ofGaP single crystal. Taking the large anisotropic etching effect of theetching solution for surface roughening into account, the surfaceroughening projections in particular in the main light extraction areaof the GaP light extraction layer can be formed by immersing the entiresurface of a flat crystalline main surface, composed of the (100)surface, of the GaP single crystal into an etching solution for surfaceroughening (that is, without forming any etching mask on the maincrystal surface composed of the (100) surface, or without forming anytrenches for exposing the {111} surfaces), and this procedure canlargely simplify the process steps, and can sufficiently enhance thelight extraction efficiency. The surface roughening projections canreadily be formed also on the side-face light extraction areas byimmersing the side faces of the layer into the etching solution forsurface roughening.

FIG. 10 is a drawing schematically explaining a concept of extraction ofincident beam from the GaP light extraction layer. Assuming therefractive index of the GaP light extraction layer as n1 (3.45 oraround), and the refractive index of a surrounding medium as n2,incident beam IB causes total reflection on the light extraction surfacewhen the angle of incidence (angle away from the normal line on thesurface) of the incident beam IB with respect to the light extractionsurface of the GaP light extraction layer reaches and exceeds a criticalangle α, and returns back into the device as reflected beam RB. Thereflected beam can escape out to the external of the layer as extractedbeam EB, only when the angle of incidence becomes smaller than thecritical angle α after repeating internal reflection. It is, however,highly probable that a considerable energy of the incident beam is lostduring this process, due to absorption and scattering inside thecrystal. The critical angle α is considerably as small as approximately16.8° when the surrounding medium is composed of the air (n2≈1), andonly as much as approximately 27.6° even if an epoxy resin mold (n2≈1.6)is used. Out of all components of incident beam incident on one point onthe light extraction surface, the components extractable out to theexternal without causing total reflection are limited to those falleninside a cone obtained by rotating, around the normal line drawn throughthe point, a generatrix inclined at an angle α away from the normalline. The cone is also referred to as an extraction cone.

On the other hand, when the light emitting layer section is understoodas a collective of a large number of point light sources arrayed inplane as shown in FIG. 11, beam of light from each of the point lightsources is emitted as spreading omni-directionally. Assuming now thenormal line drawn from each point light source down onto the lightextraction surface, it is geometrically obvious that any components ofthe beam of light emitted at angles not smaller than α, away from thenormal line, will cause angle of incidence again not smaller than α onthe light extraction surface, so far as the surface is a flat plane, sothat the light returns back into the layer by total reflection.Accordingly, as for the main light extraction area and the side-facelight extraction areas of the GaP light extraction layer, we can assumesimilar circular cones having the point light source at the apexthereof, around the normal line drawn from the point light source downonto the individual areas, so far as the surface is a flat plane. Of allcomponents of incident beam directed from each point light sourcetowards the individual areas, the components extractable out to theexternal are limited to those fallen inside the circular cones (thecircular cones are referred to as escape cones). On the other hand,formation of the surface roughening projections on the light extractionarea can largely increase the ratio of extractable beam of lightincident at low angles, when considered on the basis of substantialangle of incidence on the irregularities, and can also increase thesurface area of that area by virtue of the irregularities, so that evencomponents of the beam of light fallen outside the escape cones nowbecome extractable in an effective manner from the planar area.

Critical angle α of total reflection is a determinant of the apex angleof the extraction cone (or escape cone), wherein the angle α is as smallas 17 to 27° at most as described in the above. By considering the factthat dimensional spreading of the GaP light extraction layer is farlarger in the in-plane direction than in the thickness-wise direction,it is obvious that the ratio of area sectioned by the extraction conesstanding on the point light sources in the main light extraction areabecomes scarce, so that a large portion of the components of theincident light fall outside the extraction cones, and are therefore notextractable out to the external. The main light extraction area,therefore, shows an extremely distinctive effect of improving the lightextraction efficiency, by virtue of formation of the surface rougheningprojections.

The side-face light extraction areas have only an extremely small degreeof extension of the surface in the thickness-wise direction as comparedwith the main light extraction area, so that the extraction conesarrayed in the plane of the layer overlap with each other, and thismakes the ratio of area sectioned by the extraction cones standing onthe point light sources dense, and also makes all of four side faces,differing in the orientation, available as the light extraction areas.As a consequence, the side-face light extraction areas can achieve asufficient level of improvement in the light extraction efficiency, evenif the thoroughness of formation of the roughening projections ismoderate than in the main light extraction area. In other words, thesurface roughening projections formed on the side-face light extractionareas of the GaP light extraction layer may be formed as satisfying atleast either one of conditions that they have a smaller mean height, andthat they have a larger mean interval of formation, as compared withthose of the surface roughening projections formed in the main lightextraction area. This configuration can therefore simplify the processof formation of the surface roughening projections in the side-facelight extraction areas, raise efficiency in the manufacturing, andreduce the cost.

The side-face light extraction areas tends to cause residue ofmechanical distortions or crystal defects if they are formed by dicingor cleavage of the light emitting device wafer, and is therefore lesslikely to proceed thereon the formation of irregularities by chemicaletching as compared with the main light extraction area. Thethoroughness of formation of the surface roughening projections on theside-face light extraction areas moderated as described in the above ismore advantageous in terms of simplifying the formation process of theirregularities in the side-face light extraction areas.

If the orientation of the side-face areas of the GaP light extractionlayer is agreed with the {110} surface, which is a cleavage plane of GaPsingle crystal (allowing a degree of shift of 1° to 25°, both endsinclusive, away from the exact {110} direction, for the case where theoff-angle is given as described in the above), combination ofhalf-dicing of the wafer and breaking based on cleavage can simplify theseparation process for producing the chips, and can also contribute toimprovement in the production yield of the light emitting device,because nonconformities such as cracking in undesired directions andchipping of the chips are less likely to occur. Even in the process offull-dicing of the wafer for separation into chips, agreement of thedicing plane with the cleavage plane can improve the production yield,because the load of the dicing can be suppressed to a low level, whereinalso the chipping is less likely to occur. Aiming at full exhibition ofthe above-described advantages, it has been a fixed idea for III-Vcompound semiconductor devices having the zincblende structure, but notlimited to the light emitting device within a scope of this invention,to adjust the direction of dicing to the <110> direction when they aremanufactured by dicing wafers having the (100) main surface (also simplyreferred to as (100) wafer, hereinafter) as shown in FIG. 23. Forexample, Japanese Laid-Open Patent Publication “Tokkaihei” No. 8-115893discloses a method of fabricating a light emitting device, involvingdicing of a (100) wafer in parallel with the orientation flat, whereinthe orientation flat of the (100) wafer is generally formed in parallelwith the {110} surface, so that the dicing direction described inJapanese Laid-Open Patent Publication “Tokkaihei” No. 8-115893 is in the<110> direction.

The surface roughening projections formed by the anisotropic etchingare, however, formed as having a basic form of regular octahedronsurrounded by the {111} surfaces, and shows on the {110} surface, asshown in FIG. 6, a flattened geometry such that the regular octahedronis vertically split by a plane containing the axial line, provingintrinsic difficulty in forming deeply-profiled irregularities byanisotropic etching. Crystal defects such as dislocation caused bymechanical processing are likely to distribute along the cleavage plane,so that the surfaces exposed by the cleavage and/or dicing tend to havea relatively large density of residual dislocations or the like, andthis further retards the chemical etching. For this reason, it isunconditional in some cases that the thoroughness of formation of thesurface roughening projection on the side-face light extraction areascomposed of the {110} surface is substantially moderated. Anotherproblem resides in that the AlGaInP light emitting layer section and theGaP light extraction layer tend to produce therebetween mismatch-inducedstress due to difference in the lattice constants, so that dicing alongthe {110} surface, which is a cleavage plane, is likely to cause laminarcracks along the cleavage plane (and consequently the chip edge) undermismatch-inducing stress, and may even result in failures such aschipping of the chip edge or the like as shown in the lower part of FIG.23.

The present inventors then made extensive investigations, to finallyfind out that all of the above-described nonconformities can be resolvedif the light emitting device, having the GaP light extraction layerformed on the AlGaInP light emitting layer section, is manufacture bydicing while allowing the {100} surface to appear on the side facesthereof, as shown in FIG. 4. In other words, by avoiding agreementbetween the dicing surface and the cleavage plane, any cleaving crackseven if accidentally produced will appear in the direction crossing thechip edge as shown in the lower part of FIG. 4, and thereby the failuresascribable to chipping and the like can largely be suppressed.Thus-obtained light emitting device in this case is configured as havingthe side-face areas of the GaP light extraction layer, composed of the{100} surface of GaP single crystal.

When the side face composed of the {100} surface is further subjected toanisotropic etching using the above-described etching solution, geometryof the surface roughening projections formed in the side-face lightextraction areas shows a pyramid-like form as shown in FIG. 5, similarlyto as obtained in the main light extraction area composed of the (100)surface, allowing formation of far deeper-profiled irregularities ascompared with the embodiment shown in FIG. 6 where the {110} surfacesappear on the side faces, and this raises an additional advantage ofconsiderably improving the light extraction efficiency from the sidefaces.

Any excessive residue of the process-induced damage layer formed in theside-face light extraction areas of the GaP light extraction layer afterthe dicing not only inhibits smooth formation of the surface rougheningprojections by the succeeding chemical etching, but also allows a partof the process-induced damage layer to remain even after the formationof the surface roughening projections, possibly causative of absorptionor scattering of the beam of emitted light. It is therefore effective toetch off, after the dicing, the process-induced damage layer formed inthe side-face light extraction areas of the GaP light extraction layer,using a damage-removing solution composed of an aqueous sulfuricacid/hydrogen peroxide solution, and then to further etch it using anetching solution for surface roughening. The aqueous sulfuricacid/hydrogen peroxide solution is excellent in the effect of uniformlyetching GaP crystal including the process-induced damage layer, so thatthe process-induced damage layer in the side-face light extraction areascan thoroughly be removed prior to the etching for surface roughening,and thereby the formation of the surface roughening projections can bepromoted, and residue of the process-induced damage layer can besuppressed.

The surface roughening projections formed using the above-describedetching solution for surface roughening can be obtained in variousforms, by adjusting composition of the etching solution and etchingconditions (etching temperature and time). For example, the surfaceroughening projections can be rounded to have curved surfaces on the endportion side thereof. This geometry is obtained in a relatively earlystage of the anisotropic etching proceeded on the GaP (100) surfaceusing the above-described etching solution for surface roughening, andcan enhance the light extraction efficiency, because the angle ofincidence can be suppressed relatively small anywhere on thus-roundedcurved surface. This effect is further enhanced when each of the surfaceroughening projections has a main body composing the projection base andthinned towards the end side thereof, and a swelled tip portionintegrated with the main body, as being swelled out in a ball form onthe end side thereof.

The surface roughening projections may be formed also as having a basicgeometry obtained by anisotropic etching, and by further rounding itusing an isotropic etching solution. By this process, the outer surfaceof the surface roughening projections can have a convex curved surfacemore closer to a spherical form, and thereby the light extractionefficiency can further be improved.

A plurality of the surface roughening projections, formed as beingdistributed on the main light extraction area, may be configured ashaving a polyhedral form with a plurality of planes surrounding theouter surface of at least the projection bases, wherein the majority ofthe projections are formed as having both of φ1 and φ2 of 30° or more,satisfying φ1>φ2, where φ1 and φ2 are acute angles respectively heldbetween each of two opposing surfaces of the same projection and thefirst main surface of the GaP light extraction layer in a predetermineddirection on the main light extraction area. Use of anisotropic etchingis more likely to produce a polyhedral geometry (for example, polyhedralpyramid) based on a combination of predominant etching surfacesdiffering in the surface index (more specifically, {111} surfaces), onthe outer surface of the projection (especially on the outer surface ofthe main body composing the base side thereof). Adjustment of both of φ1and φ2, which are acute angles respectively held between each of twoopposing surfaces of the same projection and the first main surface ofthe GaP light extraction layer in the above-described predetermineddirection, to 30° or more successfully enhances an effect of reducingthe incident angle of beam of light, and contributes to improvement inthe light extraction efficiency. The light extraction efficiency furtherimproves by intentionally making difference between one angle φ1 and theother angle φ2 as described in the above.

It is now possible to configure the GaP light extraction layer as asingle crystal substrate bonded to the light emitting layer section. Inthis case, the GaP light extraction layer can readily be formed, bystacking the GaP single crystal substrate on the light emitting layersection, and annealing the stack at relatively low temperatures rangingfrom 100° C. to 700° C., both ends inclusive, to thereby bond the singlecrystal substrate directly to the light emitting layer section. On theother hand, the GaP light extraction layer can be configured also asbeing epitaxially grown on the light emitting layer section by the vaporphase growth method (for example, hydride vapor phase epitaxial growthmethod: referred to as HVPE method hereinafter).

For the case where the AlGaInP light emitting layer section is grown bythe MOVPE method, provision of an appropriate off-angle to agrowth-assisting GaAs substrate can considerably reduce ordering andmaldistribution of the Group III elements, and can yield the lightemitting device having a uniform emission spectral profile and centerwavelength. Formation of the GaP light extraction layer composed of aIII-V compound semiconductor by the HVPE method, on the alloy-base lightemitting layer section grown by the MOVPE method, scarcely produces, onthe finally-obtained GaP light extraction layer, facets and roughenedsurface ascribable to the off-angle of the GaAs substrate, and therebyyields the GaP light extraction layer excellent in the smoothness. Thiseffect is particularly distinctive when the off-angle falls within therange from 10° to 20°, both ends inclusive.

For the case where the light emitting layer section is such as beingepitaxially grown on the GaAs substrate having the principal axisthereof 1° to 25°, both ends inclusive, off-angled from the <100>direction, it is preferable to make the crystal orientation of the GaPlight extraction layer agree with thus off-angled light emitting layersection. Disagreement of the crystal orientation between the lightemitting layer section and the GaP light extraction layer may impairohmic contact between both layers, and may undesirably increase theforward voltage of the light emitting device. The GaP light extractionlayer formed by the vapor phase growth process inevitably has thecrystal orientation agreed with the crystal orientation of the lightemitting layer section, whereas for the case where the layer is formedby bonding the single crystal substrate, also the single crystalsubstrate used herein is preferably added with the same off-angle in thesame direction as the light emitting layer section.

When the light emitting layer section and the GaP light extraction layerare such as those epitaxially grown on the GaAs substrate having theprincipal axis thereof 1° to 25°, both ends inclusive, off-angled fromthe <100> direction as described in the above, anisotropic etching ofthe main light extraction area of the GaP light extraction layer usingthe above-described etching solution for surface roughening forms theprojections composed based on a combination of the {111} surfaces asbeing inclined, because the first main surface of the GaP lightextraction layer composing the main light extraction area inclines byoff-angle degrees away from the (100)_(J) surface. In short, theprojections satisfying the above-described condition φ1>φ2 can beobtained in an extremely easy manner. Assuming now that a directionconnecting the exact [100] axis and the off-angled principal crystalaxis as the above-described predetermined direction, and that anglesheld between the outer surfaces of the projection composed of two {111}surfaces opposing in this direction and the first main surface of thusoff-angled GaP light extraction layer as φ1 and φ2, provision of anoff-angle up to 25° never makes the smaller angle φ2 fall below 30°,because the angle held between the (111) surface and the (100) surfaceis approximately 55°. On the other hand, the larger angle φ1 may be aslarge as 80° or around, ensuring a steep-rising surface.

The reason why a projection 40 f satisfying φ1>φ2 can improve the lightextraction efficiency as compared with the projection satisfying φ1=φ2can be supposed as below. As shown in FIG. 9, a projection satisfyingφ1=φ2 on the (100)_(J) surface allows setting of an above-describedextraction cone EC1 on an arbitrary point on the projection. Lightincident on the extraction cone EC1 gives extracted beam EB, wherein itis assumed now that the light is reflected on the opposite surface tocome into the extraction cone EC1, and the incident beam IB on thereflection surface comes into the projection 40 f after crossing thebase surface (100)_(J) having the projection 40 f formed thereon. Inorder to obtain the incident beam IB as the reflected beam coming intothe extraction cone EC1, it is optically necessary for the incident beamIB to fall within a virtual extraction cone EC2 which is symmetricalwith the surface of the extraction cone EC1. Therefore, a problem offinding a condition allowing extraction of light through the surface ofthe above-described projection 40 f can geometrically be considered as aproblem of finding an allowable range of the incident beam IB on thebase surface (100)_(J).

The condition is that the incident beam IB falls within the extractioncone EC2, so that on the base surface (100)_(J), an area S0 sectioned bythe extraction cone EC2 can be understood as an area allowing extractionof the incident beam. When the base surface is inclined to have thesurface (100)_(OFF) due to an off-angle θ, because of a principle ofanisotropic etching the projection 40 f and consequently the extractioncone EC2 incline at θ away from the surface (100)_(OFF), while keepingthe orientation with respect to the surface (100)_(J) unchanged. Thearea sectioned by the extraction cone EC2 then changes from S0 on thesurface (100)_(J) into S1 on the surface (100)_(OFF). It isgeometrically obvious that the sectioned area becomes almost minimum(S0) on the surface (100)_(J) having the projection 40 f uprightthereon, and that the size of area (S1) on the base surface inclined bythe off-angle becomes larger than S0. In other words, the allowablerange allowing the incident beam to escape from a certain point on thesurface of the projection 40 f becomes larger in the latter case, andthis consequently contributes to improvement in the light extractionefficiency. It is to be noted that the same effect will be obtained whenthe off-angle θ is set upward with respect to the surface (100)_(J),although shown in FIG. 9 as being set downward.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing showing a sectional side elevation of anexemplary light emitting device of this invention;

FIG. 2 is a schematic plan view of the same;

FIG. 3 is a conceptual drawing of the surface roughening projectionsformed on the GaP light extraction layer shown in FIG. 1;

FIG. 4 is a drawing showing an exemplary setting of direction of dicingfor fabricating the light emitting device shown in FIG. 1, together withan effect thereof;

FIG. 5 is a conceptual drawing of a basic geometry of the surfaceroughening projection formed on the {100} base surface by anisotropicetching;

FIG. 6 is a conceptual drawing of a basic geometry of the surfaceroughening projection formed on the {110} base surface by anisotropicetching;

FIG. 7 is a first schematic drawing of the surface rougheningprojections;

FIG. 8 is a second schematic drawing of the surface rougheningprojections;

FIG. 9 is a drawing explaining a supposed principle ofinclination-induced improvement in the light extraction efficiency ofthe surface roughening projection formed on the {100}_(OFF) surface;

FIG. 10 is a drawing explaining critical angle for total reflection;

FIG. 11 is a drawing explaining difference in the light extractioneffect between the main light extraction area and the side-face lightextraction areas;

FIG. 12 is a third schematic drawing of the surface rougheningprojections;

FIG. 13 is a fourth schematic drawing of the surface rougheningprojections;

FIG. 14 is a fifth schematic drawing of the surface rougheningprojections;

FIG. 15 is a drawing explaining process steps of fabricating the lightemitting device shown in FIG. 1;

FIG. 16 is a drawing explaining process steps as continued from FIG. 15;

FIG. 17 is a drawing explaining process steps as continued from FIG. 16;

FIG. 18 is a drawing explaining process steps as continued from FIG. 17;

FIG. 19 is a schematic sectional side elevation showing a first modifiedexample of the light emitting device shown in FIG. 1;

FIG. 20 is a schematic sectional side elevation showing a secondmodified example of the light emitting device shown in FIG. 1;

FIG. 21 is a schematic sectional side elevation showing a third modifiedexample of the light emitting device shown in FIG. 1;

FIG. 22 is a schematic sectional side elevation showing another exampleof light emitting device possibly fabricated by the method of thisinvention;

FIG. 23 is a drawing showing another exemplary setting of direction ofdicing for fabricating the light emitting device shown in FIG. 1,together with a caution point;

FIG. 24 is a SEM (scanning electron microscope) observed image showing afirst exemplary observation of the surface roughening projections;

FIG. 25 is a SEM observed image showing a second exemplary observationof the surface roughening projections;

FIG. 26 is a SEM observed image showing a third exemplary observation ofthe surface roughening projections;

FIG. 27 is a SEM observed image showing a fourth exemplary observationof the surface roughening projections;

FIG. 28 is a SEM observed image showing a fifth exemplary observation ofthe surface roughening projections;

FIG. 29 is a SEM observed image showing a sixth exemplary observation ofthe surface roughening projections;

FIG. 30 is a SEM observed image showing a seventh exemplary observationof the surface roughening projections;

FIG. 31 is a SEM observed image showing an eighth exemplary observationof the surface roughening projections;

FIG. 32 is a SEM observed image showing a ninth exemplary observation ofthe surface roughening projections;

FIG. 33 is a SEM observed image showing a tenth exemplary observation ofthe surface roughening projections; and

FIG. 34 is a SEM observed image showing an eleventh exemplaryobservation of the surface roughening projections.

BEST MODES FOR CARRYING OUT THE INVENTION

Paragraphs below will describe embodiments of this invention referringto the attached drawings.

FIG. 1 is a conceptual drawing of a light emitting device 100 as oneembodiment of this invention. The light emitting device 100 has a lightemitting layer section 24 composed of a III-V compound semiconductor,and a GaP light extraction layer 20 (p-type herein) formed on a firstmain surface side of the light emitting layer section 24. On a secondmain surface side of the light emitting layer section 24, a GaPtransparent substrate 90 is disposed. In this embodiment, a chip of thelight emitting device 100 has a plain geometry of 300 μm square.

The light emitting layer section 24 has a structure in which an activelayer 5 composed of a non-doped (Al_(x)Ga_(1-x))_(y)In_(1-y)P alloy(where, 0≦x≦0.55, 0.45≦y≦0.55) is held between a p-type cladding layer(first-conductivity-type cladding layer) 6 composed of a p-type(Al_(z)Ga_(1-z))_(y)In_(1-y)P alloy (where, x<z≦1), and an n-typecladding layer (second-conductivity-type cladding layer) 4 composed ofan n-type (Al_(z)Ga_(1-z))_(y)In_(1-y)P alloy (where, x<z≦1). The lightemitting device 100 shown in FIG. 1 has the p-type AlGaInP claddinglayer 6 disposed on the first main surface side (upper side in thedrawing), and has the n-type AlGaInP cladding layer 4 disposed on thesecond main surface side (lower side in the drawing). It is to be notedthat “non-doped” in the context herein means “not intentionally addedwith a dopant”, and never excludes possibility of any dopant componentsinevitably included in the normal fabrication process (up to 1×10¹³ to1×10¹⁶/cm³ or around, for example). The light emitting layer section 24is grown by the MOVPE method. Thickness of each of the n-type claddinglayer 4 and the p-type cladding layer 6 is typically 0.8 μm to 4 μm(more preferably 0.8 μm to 2 μm), both ends inclusive, and thickness ofthe active layer 5 is typically 0.4 μm to 2 μm (more preferably 0.4 μmto 1 μm), both ends inclusive. Total thickness of the light emittinglayer section 24 as a whole is typically 2 μm to 10 μm (more preferably2 μm to 5 μm), both ends inclusive.

The GaP light extraction layer 20 is formed to as thick as 10 μm to 200μm (more preferably 40 μm to 200 μm: typically 100 μm in thisembodiment), both ends inclusive, and as shown in FIG. 2, alight-extraction-area-side metal electrode 9 is formed so as to cover apart (center portion herein) of the first main surface thereof. Thelight-extraction-area-side metal electrode 9 is connected with one endof an electrode wire 17. The peripheral area around thelight-extraction-area-side metal electrode 9 forms a main lightextraction area 20 p. The side faces of the GaP light extraction layer20 form side-face light extraction areas 20S. The GaP light extractionlayer 20 formed to as thick as described in the above allows emissiondrive current, based on current supply through thelight-extraction-area-side metal electrode 9, to spread in-plane overthe device, to thereby function as a current spreading layer assistingin-plane uniform emission of the light emitting layer section 24, andalso takes part in increasing beam of light extractable from the sidefaces of the layer, to thereby function as raising the luminance of thedevice as a whole (integrating sphere). GaP has a band gap energy largerthan that of AlGaInP composing the active layer 5, and is desirablysuppressed in absorption of the emission beam.

The GaP light extraction layer 20 in this embodiment is grown by theHVPE method (MOVPE method also allowable). Between the GaP lightextraction layer 20 and the light emitting layer section 24, aconnection layer 20J composed of a GaP layer is formed, as beingcontinued from the light emitting layer section 24 by the MOVPE method.The connection layer 20J herein may be composed of an AlGaInP layercapable of gradually changing difference in the lattice constants (andconsequently alloy composition) between the light emitting layer section24 composed of AlGaInP and the GaP light extraction layer 20. The GaPlight extraction layer 20 may be formed also by bonding of a GaP singlecrystal substrate, in place of being formed as an epitaxially-grownlayer by the HVPE method.

The GaP transparent substrate 90 is formed by bonding a GaP singlecrystal substrate (an epitaxially-grown layer obtained by the HVPEmethod also allowable: reference numeral 91 represents a connectionlayer composed of AlGaInP), and the entire range of the second mainsurface is covered with a back electrode 15 composed of an Au electrodeor the like. The crystal orientation of the GaP transparent substrate 90is agreed with that of the light emitting layer section 24 (that is,degrees of the off-angle coincide). Thickness of the GaP transparentsubstrate 90 is typically 10 μm to 200 μm, both ends inclusive. The backelectrode 15 also functions as a reflective layer for the beam of lightcoming from the light emitting layer section 24 through the GaPtransparent substrate 90, and contributes to improvement in the lightextraction efficiency. Between the back electrode 15 and the GaPtransparent substrate 90, a bond-assisting alloyed layer 15 c composedof an AuBe alloy or the like, for reducing contact resistance betweenthe both, is formed as being distributed according to a dot pattern. Thebond-assisting alloyed layer 15 c slightly lowers its reflectivity asalloying with a compound semiconductor layer composing the GaPtransparent substrate 90 proceeds, so that the layer is formed as beingdistributed according to a dot pattern, so as to allow the backgroundarea to function as a direct reflection surface composed of ahigh-reflectivity back electrode 15. On the other hand, a bond-assistingalloyed layer 9 a composed of an AuGeNi alloy or the like is formedbetween the light-extraction-area-side metal electrode 9 and the GaPlight extraction layer 20. Both of the GaP light extraction layer 20 andthe GaP transparent substrate 90 are respectively adjusted to have adopant concentration of 5×10¹⁶/cm³ to 2×10¹⁸/cm³ (dopant concentrationherein is defined excluding any high-concentration doped region forraising contact resistance, occasionally formed directly under thebond-assisting alloyed layer 9 a).

The GaP light extraction layer 20 has, as shown in FIG. 3, surfaceroughening projections 40 f, 50 f formed by chemical etching on the mainlight extraction area 20 p and on the side-face light extraction areas20S, respectively. The main light extraction area (first main surface)20 p of the GaP light extraction layer 20 has a base plane, obtained byleveling the irregularities, as being approximately agreed with the(100) surface of the GaP light extraction layer 20 (where, off-angle of1° to 25°, and 15° in this embodiment, is provided as described later),and the surface roughening projections 40 f are formed by bringing theflat (100) principal crystal surface into contact with an etchingsolution for surface roughening described later so as to proceedanisotropic etching. Also the side-face light extraction areas 20S arenearly agreed with the {100} surfaces, and the surface rougheningprojections 50 f are formed similarly by anisotropic etching. By formingthe surface roughening projections 40 f, 50 f, the light emitting device100 is largely improved in the light extraction efficiency,synergistically with an effect of increase in the side face area byvirtue of increase in the thickness of the GaP light extraction layer20.

The outer surfaces of projection composing the surface rougheningprojections 40 f, 50 f are formed by chemical anisotropic etching of GaPsingle crystal, as being mainly composed of the {111} surfaces (50% ormore of the total surface of the projections). The surface rougheningprojections 40 f, 50 f have a mean height of 0.1 μm to 5 μm, both endsinclusive, and a mean interval of formation of the projections of 0.1 μmto 10 μm, both ends inclusive. In the side-face light extraction areas20S, thoroughness of formation of the surface roughening projections 50f is moderated as compared in the main light extraction area 20 p. Morespecifically, the surface roughening projections 50 f formed in theside-face light extraction areas 20S satisfy at least either one ofconditions that they have a smaller mean height as compared with that ofthe surface roughening projections 40 f formed in the main lightextraction area 20 p (h2<h1 in FIG. 3), and that they have a larger meaninterval of formation (δ2>δ1 in FIG. 3). This configuration can furthersimplify the process of formation of the surface roughening projections50 f on the side-face light extraction areas 20S, raise efficiency inthe manufacturing, and reduce the cost.

Paragraphs below will describe a method of fabricating the lightemitting device 100 shown in FIG. 1.

First as shown in STEP 1 of FIG. 15, an n-type GaAs single crystalsubstrate 1, having an off-angle θ of 1° to 25°, both ends inclusive,(15° in this embodiment) is obtained as a growth-assisting substrate.Next, as shown in STEP 2, an n-type GaAs buffer layer 2 of typically 0.5μm thick is epitaxially grown on the main surface of the substrate 1,and further thereon, as the light emitting layer section 24, the n-typecladding layer 4 of 1 μm thick (n-type dopant is Si), the active layer(non-doped) 5 of 0.6 μm thick, and the p-type cladding layer 6 of 1 μmthick (p-type dopant is Mg: also C derived from organo-metallicmolecules can be contributive as the p-type dopant), respectivelycomposed of an (Al_(x)Ga_(1-x))_(y)In_(1-y)P alloy are epitaxially grownin this order. The dopant concentrations of the p-type cladding layer 6and the n-type cladding layer 4 typically fall in the range from1×10¹⁷/cm³ to 2×10¹⁸/cm³, both ends inclusive. Further on the p-typecladding layer 6, the connection layer 20J is epitaxially grown as shownin STEP 3 of FIG. 16.

The above-described individual layers are epitaxially grown by anypublicly-known MOVPE method. Source gases available as sources of theindividual components Al, Ga, In (indium) and P (phosphorus) include thefollowings:

Al source gas: trimethyl aluminum (TMAl), triethyl aluminum (TEAl),etc.;

Ga source gas: trimethyl gallium (TMGa), triethyl gallium (TEGa), etc.;

In source gas: trimethyl indium (TMIn), triethyl indium (TEIn), etc.;and

P source gas: trimethyl phosphorus (TMP), triethyl phosphorus (TEP),phosphine (PH₃), etc.

The process then advances to STEP 4 in FIG. 16, wherein the GaP lightextraction layer 20 composed of p-type GaP is grown by the HVPE method.The HVPE method is specifically proceeded so that Ga as a Group IIIelement is heated and kept at a predetermined temperature in a vessel,and hydrogen chloride is introduced over Ga to thereby produce GaClthrough a reaction expressed by the formula (1) below, and is thensupplied over the substrate together with H₂ gas as a carrier gas:Ga(l)+HCl(g)→GaCl(g)+½H₂  (1)The growth temperature is typically set to 640° C. to 860° C., both endsinclusive, wherein P as a Group V element is supplied on the substratein a form of PH₃, together with H₂ as a carrier gas. Zn as a p-typedopant is supplied in a form of DMZn (dimethyl Zn). GaCl is excellent inreactivity with PH₃, and can efficiently grow the GaP light extractionlayer 20 according to the reaction expressed by the formula (2) below:GaCl(g)+PH₃(g)→GaP(s)+HCl(g)+H₂(g)  (2)

After completion of the growth of the GaP light extraction layer 20, theprocess advances to STEP 5 in FIG. 17, wherein the GaAs substrate 1 isremoved by chemical etching using an etching solution such as anammonia/hydrogen peroxide mixed solution. On the second main surfaceside of the light emitting layer section 24 (second main surface of aconnection layer 91), from which the GaAs substrate 1 has already beenremoved, a separately-obtained, n-type GaP single crystal substrate isbonded to thereby obtain the GaP transparent substrate 90 (STEP 6).

After completion of the above-described process steps, metal layers forforming the bond-assisting alloyed layers are formed respectively on thefirst main surface of the GaP light extraction layer 20 and on thesecond main surface of the GaP transparent substrate 90 by a sputteringor a vacuum deposition method as shown in STEP 7 in FIG. 18, and thenannealed for alloying (so-called sintering) so as to form thebond-assisting alloyed layers 9 a, 15 c (see FIG. 1, not shown in FIG.18). The light-extraction-area-side electrode 9 and the back electrode15 are formed so as to cover the bond-assisting alloyed layers 9 a and15 c, respectively, to thereby obtain a light emitting device wafer W.

Next, as shown in STEP 8, the main light extraction area ((100) mainsurface) of the GaP light extraction layer 20 is anisotropically etchedusing an etching solution FEA for surface roughening, to thereby formthe surface roughening projections 40 f. The etching solution forsurface roughening is an aqueous solution containing acetic acid,hydrofluoric acid, nitric acid and iodine, and specifically contains:

acetic acid (on CH₃COOH basis): 37.4% by mass or more and 94.8% by massor less;

hydrofluoric acid (on HF basis): 0.4% by mass or more and 14.8% by massor less;

nitric acid (on HNO₃ basis): 1.3% by mass or more and 14.7% by mass orless; and

iodine (on I₂ basis): 0.12% by mass or more and 0.84% by mass or less,

and has a water content of 2.4% by mass or more and 45% by mass or less,and more preferably contains:

acetic acid (on CH₃COOH basis): 45.8% by mass or more and 94.8% by massor less;

hydrofluoric acid (on HF basis): 0.5% by mass or more and 14.8% by massor less;

nitric acid (on HNO₃ basis): 1.6% by mass or more and 14.7% by mass orless; and

iodine (on I₂ basis): 0.15% by mass or more and 0.84% by mass or less,

and has a water content of 2.4% by mass or more and 32.7% by mass orless. The solution temperature is appropriately adjusted to 40° C. to60° C., both ends inclusive.

With progress of the anisotropic etching, the surface rougheningprojections formed on the flat (100) main surface of GaP gradually addsthe depth of formation of the side faces of the pyramids composed of the{111} surface, while shrinking a flat area 40 p between the projections,as shown in FIG. 12 and FIG. 13. In the early stage of this process, theprojections show a rounded geometry in a form of having curved surfaces40 r on the end side thereof, as shown in FIG. 12. FIG. 24 and FIG. 25are images observed under scanning electron microscope (×5000), showinga specific example of the formation, wherein FIG. 24 shows a perspectiveimage, and FIG. 25 shows a plan image. The etching solution used hereincontains 81.7% by mass of acetic acid, 5% by mass of hydrofluoric acid,5% by mass of nitric acid and 0.3% by mass of iodine, with a watercontent suppressed to as low as 8% by mass. The solution temperature is50° C. and etching time is 60 seconds. Assuming now the angle heldbetween the curved surface 40 r and the tangential plane as the angle ofincidence of beam of emitted light, this geometry ensures a relativelylarge angle of incidence anywhere on the curved surface 40 r, and canconsequently raise the light extraction efficiency. The flat area 40 pappropriately remaining between the projections makes it less likely tocause re-incidence of the beam of emitted light once extracted from oneprojection into the adjacent projection.

On the other hand, as the etching proceeds, the projections not onlyadds their height as shown in FIG. 13, but also causes changes in thegeometry, appeared as having a main body 40 w composing the base side ofthe projection and thinned towards the end side thereof, and a swelledtip portion 40 s integrated with the main body 40 w, as being swelledout in a ball form on the end side thereof. This geometry increasesratio of the inclined {111} surfaces composing the outer surface of themain body 40 w, and the ball form of the swelled tip portion 40 sapproaches a sphere which is ideal for light extraction, ensuring a evenbetter light extraction efficiency. FIG. 26 and FIG. 27 are imagesobserved under scanning electron microscope, showing other specificexamples of the formation, wherein FIG. 26 shows a perspective image(×5000), and FIG. 27 shows a sectional side elevation (×20000). Theetching solution used herein contains 81.7% by mass of acetic acid, 5%by mass of hydrofluoric acid, 5% by mass of nitric acid and 0.3% by massof iodine, with a water content suppressed to as low as 8% by mass. Thesolution temperature is 50° C. and etching time is 90 seconds.

When the etching further proceeds thereafter, the swelled tip portionsdisappear as shown in FIG. 8, and almost entire portion of the sidefaces of the projections are occupied by the {111} surfaces, comingcloser to a geometry of steeply-pointed pyramid (see also FIG. 5). Thisstate ensures a largest density of formation and a largest height of theprojections, and therefore realizes an excellent light extractionefficiency. FIG. 28 and FIG. 29 are images observed under scanningelectron microscope, showing other specific examples of the formation,wherein FIG. 28 shows a perspective image (×20000), and FIG. 29 shows asectional side elevation (×20000). The etching solution used hereincontains 81.7% by mass of acetic acid, 5% by mass of hydrofluoric acid,5% by mass of nitric acid and 0.3% by mass of iodine, with a watercontent suppressed to as low as 8% by mass. The solution temperature is50° C. and etching time is 120 seconds.

When the (100)_(J) surface appears on the first main surface of the GaPlight extraction layer 20 (that is, when the off-angle θ is 0° in STEP 1of FIG. 15), the obtained projections have a nearly uprightsemi-octahedral geometry as shown in FIG. 7, having angles φ1 and φ2held between each of two opposing side faces and the (100)_(J) surfaceequal to each other (φ1=φ2=φ0; approximately 55°). However underprovision of off-angle θ as shown in FIG. 8, also the (100)_(J) surfacesincline at an angle of θ away from the first main surface ((100)_(OFF))of the GaP light extraction layer 20. As a consequence, in the directionof connecting the [100]_(J) axis and the normal line on the first mainsurface of the GaP light extraction layer 20, angles φ1′ and φ2′ heldbetween each of two opposing side faces and the (100)_(OFF) surfaceappear that angle φ1′ located in the direction of inclination of thenormal line on the (100)_(J) is larger than angle φ2′ on the oppositeside. This configuration contributes further improvement in the lightextraction efficiency. When considered on the basis of angle φ0 under anoff-angle θ of 0°, provision of off-angle θ gives φ1′=φ0+θ and φ2′=φ0−θ.An off-angle θ of 15°, for example, gives φ1′=70° or around and φ2′=40°or around, both being larger than 30°.

Referring now back to FIG. 18, after completion of formation of thesurface roughening projections 40 f in the main light extraction area,the wafer W is then diced in two <100> directions (where, inclinationnot larger than 25°, and preferably not larger than 15°, away from the<100>_(J) surface also allowable) from the first main surface sidethereof, using a dicing blade so as to form grooves DG forindividualizing the chips. The direction of dicing agreed with the <100>direction makes cracking or chipping along the edges of the chip arealess likely to occur. In the dicing, a process-induced damage layer 20Dhaving a relatively high density of crystal defects is formed as shownin STEP 9 of FIG. 18. A large number of crystal defects contained in theprocess-induced damage layer 20D are causative of current leakage andscattering during current supply for light emission, so that, as shownin STEP 10, the process-induced damage layer 20D is removed by chemicaletching using an etching solution DEA for damage removal. The etchingsolution DEA for damage removal used herein is an aqueous sulfuricacid/hydrogen peroxide solution. The aqueous solution adoptable hereinis such as having a ratio-by-mass of mixing of (sulfuric acid):(hydrogenperoxide):(water)=20:1:1, and the solution temperature is adjusted to30° C. to 70° C., both ends inclusive.

The dicing direction may be set also in two <110> directions normal toeach other, on the first main surface ((100) surface) of the wafer W asshown in FIG. 23. In this case, the side faces exposed by the dicing aregiven as the {110} surfaces, that is, given in a form agreed with thecleavage plane of the zincblende-type, III-V compound semiconductorcrystal. By this setting, the individual chips 100C become less likelyto cause chipping, and thereby the yield can be improved. It is,however, intrinsically difficult on the {110} surface to dramaticallyincrease the height of the projections mainly composed of the {111}surfaces, as shown in FIG. 6, so that formation of the surfaceroughening projections using the above-described etching solution isless likely to proceed.

Thereafter, as shown in FIG. 11, the side faces of the chip afterremoval of the process-induced damage layer 20D is brought into contactwith the etching solution FEA for surface roughening so as toanisotropically etch the side faces of the GaP light extraction layer20, to thereby form the surface roughening projections 50 f. In thisembodiment, the wafer W is bonded to a base 60 while placing an adhesivesheet 61 in between, and fully diced in depth, so that the surfaceroughening projections 50 f are formed also on the side faces of the GaPtransparent substrate 90.

The side faces of the chip after the dicing may sometimes have aresidual stress layer 20δ remained therein even after removal of theprocess-induced damage layer, and may thereby make it difficult toproceed thereon the anisotropic etching using the etching solution FEAfor surface roughening. Whereas by dicing the wafer so as to expose the{100} surfaces on the side faces as shown in FIG. 4, distinctiveprojections can be formed as shown in FIG. 30 and FIG. 31 (FIG. 30 is aplan image at a 5000× magnification, and FIG. 31 is a perspective imageat a 10000× magnification), although showing a slightly lower etchrateas compared with the main surface not affected by the dicing.

When it is desired to avoid influence of the etching possibly exerted onthe main light extraction area 20 p having the surface rougheningprojections 40 f already formed thereon, in the process of forming thesurface roughening projections 50 f in the side-face light extractionareas 20S, it is preferable to mask the main light extraction area 20 pwith an etching resist 20M, as indicated by a dashed line in STEP 9 toSTEP 11 of FIG. 18. It is also allowable to dice the wafer before thesurface roughening projections 40 f are formed on the main lightextraction area 20 p, and to form the surface roughening projections 40f and 50 f at the same time in the main light extraction area 20 p andin the side-face light extraction areas 20S.

In both of the main light extraction area 20 p and the side-face lightextraction areas 20S, the surface roughening projections 40 f and 50 fmay be such as those having a basic geometry 40 f′ (50 f′) obtained byanisotropic etching as shown in FIG. 14, and further rounded using anisotropic etching solution, to thereby obtain the final forms of thesurface roughening projections 40 f (50 f). The isotropic etchingsolution applicable herein is an aqueous sulfuric acid/hydrogen peroxidesolution, similarly to the above-described etching solution for damageremoval. FIG. 32 is a plan image showing an exemplary formation at a5000× magnification, FIG. 33 is a perspective image at a 10000×magnification, and FIG. 34 is a perspective image at a 2000×magnification. Etching conditions involve an etching solution containing81.7% by mass of acetic acid, 5% by mass of hydrofluoric acid, 5% bymass of nitric acid and 0.3% by mass of iodine, with a water contentsuppressed to as low as 8% by mass, a solution temperature of 50° C.,and an etching time of 120 seconds for the preceding anisotropicetching; and an aqueous solution having a ratio-by-mass of mixing of(sulfuric acid):(hydrogen peroxide):(water)=20:1:1, a solutiontemperature of 50° C., and an etching time of 150 seconds for thesucceeding isotropic etching.

The separated light emitting device chip is bonded, on the second mainsurface side thereof, to a metal stage while placing an Ag paste layerin between, followed by, as shown in FIG. 1, connection of a bondingwire 9 w to the light-extraction-side electrode 9, and formation of anunillustrated mold portion composed of an epoxy resin, to therebycomplete a final form of the light emitting device.

Paragraphs below will describe various modified examples of the lightemitting device of this invention (any components identical to those inthe light emitting device 100 shown in FIG. 1 will be given with thesame reference numerals so as to avoid detailing, and only differentaspects will be explained). A light emitting device 200 shown in FIG. 19is configured as having a metal reflective layer 10 composed of Au or Ag(or any alloys having these elements as major components), in place ofbonding the GaP transparent substrate 90 on the second main surface sideof the light emitting layer section 24 of the light emitting device inFIG. 1. Beam of light emitted from the light emitting layer section 24is reflected on the metal reflective layer 10 back towards the mainlight extraction area side, successfully realizing a light emittingdevice having a strong directionality on the main light extraction areaside. In this embodiment, an electro-conductive Si substrate 7 is bondedto the second main surface of the light emitting layer section 24, whileplacing the metal reflective layer 10 in between. The Si substrate 7 hasthe back electrode 15 formed on the second main surface side thereof,wherein a bond-assisting layer 15 d is formed in between over the entiresecond main surface of the Si substrate 7, because the back electrode 15does not form a reflective surface. Between the metal reflective layer10 and the light emitting layer section 24, a dot-patterned,bond-assisting alloyed layer 32 (typically composed of an AuGeNi alloy)is formed in a distributed manner.

A light emitting device 300 exemplified in FIG. 20 dare uses the opaqueGaAs substrate 1 directly as a device-forming substrate, rather thanremoving it. A light emitting device 400 exemplified in FIG. 21 has theGaAs substrate 1 cut off in the circumferential portion thereof so as toexpose the circumferential portion of the light emitting layer section24 on the second main surface side thereof, allowing extraction of lightalso through the exposed portion.

A light emitting device 500 shown in FIG. 22 is thinned as a whole so asto enhance heat radiation effect, by setting the thickness of the GaPlight extraction layer 20 to less than 40 μm, and further to asrelatively small as 5 μm to 30 μm, both ends inclusive. The GaP lightextraction layer 20 has the surface roughening projections 40 f formedonly on the main light extraction area 20 p thereof, and has no surfaceroughening projections formed on the side faces thereof, because the GaPlight extraction layer 20 has only a small thickness and shows only aless distinctive effect of allowing light extraction from the sidefaces, as compared with the light emitting device 100 shown in FIG. 1.In the process steps of fabricating thus-configured chip shown in FIG.18, the formation of the surface roughening projection on the side facesshown in STEP 11 is omissible.

1. A method of fabricating a light emitting device comprising: a lightemitting device wafer fabricating step fabricating a light emittingdevice wafer having a light emitting layer section based on a doubleheterostructure in which a first-conductivity-type cladding layer, anactive layer and an second-conductivity-type cladding layer, each ofwhich being composed of a compound semiconductor having a compositionallowing lattice matching with GaAs, out of compound semiconductorsexpressed by formula (Al_(x)Ga_(1-x))In_(1-y)P (where 0≦x≦1, 0≦y≦1), arestacked in this order, and a GaP light extraction layer disposed on saidlight emitting layer section so as to allow a first main surface thereofto compose a first main surface of the wafer, so that the first mainsurface of said GaP light extraction layer appears as the (100) surface;a main light extraction area roughening step forming surface rougheningprojections on the first main surface of said GaP light extraction layercomposed of said (100) surface, by etching said surface using an etchingsolution for surface roughening containing acetic acid, hydrofluoricacid, nitric acid, iodine and water up to a total content of 90% by massor more, and having a total content by mass of acetic acid, hydrofluoricacid, nitric acid and iodine larger than the content by mass of water;and a dicing step dicing said light emitting device wafer to therebyfabricate light emitting device chips each having said surfaceroughening projections on the first main surface of said GaP lightextraction layer.
 2. The method of fabricating a light emitting deviceas claimed in claim 1, wherein said etching solution for surfaceroughening contains: acetic acid (on CH₃COOH basis): 37.4% by mass ormore and 94.8% by mass or less; hydrofluoric acid (on HF basis): 0.4% bymass or more and 14.8% by mass or less; nitric acid (on HNO₃ basis):1.3% by mass or more and 14.7% by mass or less; and iodine (on I₂basis): 0.12% by mass or more and 0.84% by mass or less, and has a watercontent of 2.4% by mass or more and 45% by mass or less.
 3. The methodof fabricating a light emitting device as claimed in claim 1, whereinsaid etching solution for surface roughening contains: acetic acid (onCH₃COOH basis): 45.8% by mass or more and 94.8% by mass or less;hydrofluoric acid (on HF basis): 0.5% by mass or more and 14.8% by massor less; nitric acid (on HNO₃ basis): 1.6% by mass or more and 14.7% bymass or less; and iodine (on I₂ basis): 0.15% by mass or more and 0.84%by mass or less, and has a water content of 2.4% by mass or more and32.7% by mass or less.
 4. The method of fabricating a light emittingdevice as claimed in claim 1, wherein said GaP light extraction layer isformed to be as thick as 10 μm or more on said light emitting devicewafer, and further comprising a side-face light extraction arearoughening step forming surface roughening projections on the side-facelight extraction areas of said GaP light extraction layer composed ofthe side faces of the chips formed by said dicing, by etching said areasusing said etching solution for surface roughening.
 5. The method offabricating a light emitting device as claimed in claim 4, wherein saiddicing is carried out so that the {100} surface appears on the sidefaces of the chip.
 6. The method of fabricating a light emitting deviceas claimed in claim 4, wherein said surface roughening projections areformed by removing, after said dicing step, a process-induced damagelayer formed in said side-face light extraction areas of said GaP lightextraction layer using an etching solution for removing damage composedof an aqueous sulfuric acid/hydrogen peroxide solution, and then byetching the exposed surface by said etching solution for surfaceroughening.
 7. The method of fabricating a light emitting device asclaimed in claim 6, wherein said GaP light extraction layer is formed tobe as thick as 40 μm or more.
 8. The method of fabricating a lightemitting device as claimed in claim 1, wherein said GaP light extractionlayer is formed to be thinner than 40 μm, and said surface rougheningprojections are not formed on the side faces of said GaP lightextraction layer.
 9. The method of fabricating a light emitting deviceas claimed in claim 1, wherein said surface roughening projectionsformed by an anisotropic etching using said etching solution for surfaceroughening are further rounded by etching using an isotropic etchingsolution.